(A) Field of the Invention
The present invention is related to a track-locking method and the relevant implementation apparatus for an optical disk drive, more specifically, to a track-locking method and the relevant implementation apparatus for an optical disk drive by reshaping tracking error signals.
(B) Description of the Related Art
In the design of the control chip for an optical disk drive, the track-locking of the pickup head of the optical disk drive is a very important control mechanism. Regardless of the manners of so-called static tracking-on or the track-locking after seeking, they all directly affect the data accessing speed and quality of the optical disk drive.
Based on the principle that the optical disk has different optical reflectivity for the on-track period and the off-track period, the optical disk drive utilizes a pickup head to emit a light beam onto the optical disk, and uses an optical sensor to detect the intensity variance of the reflected lights, so as to obtain the so-called tracking error (TE) signal, and determine whether the position of the pickup head is correct. With respect to the different positions of the pickup head, the TE signal can be linked as a curve similar to a sine wave as shown in the upper part of FIG. 1, where the on-track periods 1 and the off-track periods 2 are arranged alternatively and separated by dotted lines.
However, the TE signal generated under such structure will decrease gradually during the seek periods, i.e., the TE signal decreases gradually in the off-track periods 2 and cannot reflect the situation that the pickup head is moving away from the track. Therefore, the TE signal in the off-track periods cannot be directly employed to reflect the actual position of the pickup head.
To resolve the above-described problem, holding the extreme values of the TE signal from an on-track period to an off-track period is a known control method to prevent signal from decreasing gradually along the original sine wave, which is shown as the corrected tracking error signal in the lower part of FIG. 1. As a result, the pickup head of the optical disk drive can be positioned at the object track.
However, such peak holding method still has disadvantages, that is, when the rotation speed of the optical disk drive is relatively high, e.g., higher than 5000 rpm, or the eccentricity is relatively large, e.g., larger than 70 μm, the required control force is relatively large. Hence, the above-described method cannot provide appropriate large error amount in the off-track periods to obtain stronger control force so that it is liable to have a track-sliding issue and it is not easy to obtain an ideal control performance, resulting in needing longer track-locking time. Besides, the intervals between peaks of the TE signal are limited by the bandwidth, inducing decreases in the TE signal when the seeking speed is relatively fast. Usually, when performing the seek process, the seeking speed will be controlled to be relatively slow at the beginning and the end, and relatively fast in the middle, so the shape of the TE signal during the entire seek process shall be similar to a dog bone. However, with the development of high speed-factor and the requirements for higher performance, the seeking speed is increased. When the seeking process ends, a seeking speed that is too high may result in the signal reduction, so the TE signal during track-locking period cannot correctly reflect the deviation of the optical point of the pickup head from the track. Therefore, the track-sliding phenomenon may occur easily.